Stall detection and recovery system

ABSTRACT

A stall detection and recovery system ( 10 ) for a gas turbine engine ( 1 ) comprises surge detection means ( 50  for detecting an engine surge and providing a surge detection signal ( 154 ) for indicating said engine surge. The system also includes stall detection means to provide a stall detection signal ( 48 ) relating to a condition of gas flowing through the engine ( 1 ). The system ( 10 ) also includes stall recovery means ( 58 ) to receive the surge detection signal ( 154 ) and the stall detection signal. The stall recovery means ( 58 ) is arranged to control combustor operating means, whereby when the stall detection signal ( 48 ) indicates an engine stall, the stall recovery means controls the combustor operating means to effect recover from the stall.

This invention relates to stall detection and recovery systems for gasturbine engines. The invention also relates to stall detection means.

In gas turbine engines used on aircraft, there is a risk of externallyinduced surge and stall events, for example due to aircraft manoeuvres.These events can be particularly serious on single engine installations,if they occur during critical flight phases, for example near theground.

There are three possible outcomes from a gas turbine engine surge:—

(1) natural recovery

(2) stall

(3) immediate flameout.

Stall is a quasi-steady state that can develop in an engine following asurge. It is characterised by low efficiency and flow capacity and canresult in engine overheat and large scale thrust loss.

In many gas turbine engines, there is a power level above which naturalrecovery is the normal outcome and below which stall is the normaloutcome. Immediate flameout usually only occurs at low pressures and/orweak initial fuel mixtures.

Control actions in order to recover from a stall are normally initiatedupon the detection of the initial surge event. There is, thus, thepossibility that unnecessary control action could be taken in the caseof a naturally recoverable surge. It is undesirable to trigger thecontrol action to recover from a stall in the case of a naturallyrecoverable surge.

According to one aspect of this invention there is provided a stalldetection and recovery system for a gas turbine engine, the systemcomprising surge detection means for detecting an engine surge andproviding a surge detection signal indicating said engine surge, stalldetection means to provide a stall detection signal relating to acondition of gas flowing through the engine, and stall recovery means toreceive said surge detection signal and said stall detection signal,said stall recovery means being arranged to control combustor operatingmeans, whereby when the stall detection signal indicates an enginestall, the stall recovery means controls the combustor operating meansto effect recovery from the stall.

Preferably, the combustor operating means comprises means to supply fuelto the combustor, and may also comprise an ignitor for igniting thefuel.

Preferably, the control means operates to interrupt momentarily saidsupply of fuel and to ignite the fuel supplied to the engine after saidmomentary interruption.

Preferably, the condition of the gas to which the second signal relatesis pressure of the gas.

Preferably, the system includes delay means to impart a delay of apredetermined time to the surge detection signal. Desirably, the stallrecovery means operates to effect recovery from the stall saidpredetermined time after the surge detection signal indicates an enginesurge providing the stall detection signal still indicates an enginestall. The predetermined time may be 0 to 10 seconds, preferably 0.1 to10 seconds, more preferably 0.1 to 1 second and, most preferably,substantially 0.5 seconds.

The stall detection means may comprise a pressure monitoring arrangementfor monitoring the pressure at a region of the engine and providing apressure signal. Preferably, the stall detection means comprisesprocessing means to process the pressure signal. The pressure monitoringarrangement may comprise a first pressure measuring means for measuringthe pressure at a first region of the engine and providing a measuredfirst signal, being said pressure signal. The pressure monitoringarrangement may comprise a second pressure measuring means for measuringthe pressure at a second region of the engine and providing a measuredsecond pressure signal. Preferably, the second region of the engine isthe combustor, and the second pressure signal desirably relates to thepressure in the combustor.

The processing means may comprise filter means for filtering the firstpressure signal. Preferably, the filter means provides a first filteredsignal indicative of the time average value of the first pressuresignal. The filter means may provide a second filtered signal indicativeof high values of the first pressure signal. The second filtered signalmay be indicative of maximum peaks of the first pressure signal. Thefilter means may provide a third filtered signal indicative of lowvalues of the first pressure signal. The third filtered signal may beindicative of minimum peaks of the first pressure signal.

The processing means may further include comparator means for providinga first difference signal, being the difference between the firstfiltered signal and the second filtered signal. The comparator means mayalso provide a second difference signal, being the difference betweenthe first filtered signal and the third filtered signal.

The processing means may further include gate means, for transmittingthe lowest of the first and second difference signals. The gate means ispreferably a low wins gate.

The processing means may further comprise multiplying means formultiplying said transmitted difference signal by a predeterminedfactor, and

provide a multiplied pressure signal. Conveniently, the predeterminedfactor is 2.

The stall detection means may comprise fixed signal transmission meansfor transmitting a predetermined second pressure signal. Thepredetermined second pressure signal is preferably a signal relating toa pressure which is lower than the expected pressure of the secondpressure signal during normal running of the engine. Preferably, thepredetermined second pressure signal is higher than the pressure in thesecond region of the engine during engine start or sub-idle operation orwhen the engine is unlit. For example, the predetermined second pressuresignal may relate to a pressure of substantially 10 psia.

The stall detection means may comprise second processing means toprocess the measured second pressure signal, and the predeterminedsecond pressure signal. The second processing means may comprise furthergate means for transmitting the highest of the measured second pressuresignal and the predetermined second pressure signal. The further gatemeans may be a high wins gate.

Preferably, stall detection means comprises divider means to divide themultiplied pressure signal by the transmitted second pressure signal toprovide a divided signal.

The stall detection means may further comprise a comparator device tocompare the divided signal by a threshold signal, whereby in the eventof the divided signal being greater than the threshold signal, a stallis indicated. Preferably, the comparator device provides said stalloutput signal. The stall detection means may comprise integration means,which may be a fault integrator. The stall output signal may be receivedby the integration means. Preferably, the integration means can producesaid stall detection signal which indicates that a stall has beendetected.

The surge detection means may provide a surge signal, indicative of asurge having been detected. Preferably, the surge signal is received bythe delay means to imparts said delay to the surge signal.

The surge detection means may comprise combustor monitoring means tomonitor a condition of the gas in the combustor. Preferably, thecombustor monitoring means monitors combustor pressure.

Preferably, the stall detection signal and the surge detection signalare transmitted through the control means for controlling the supply offuel and the ignitor as aforesaid.

According to another aspect of this invention, there is provided stalldetection means for a gas turbine engine, comprising a monitoringarrangement for monitoring the condition of gas flowing through theengine at a first region of the engine and providing a first pressuresignal, and for monitoring the condition of gas flowing through theengine at a second region of the engine and providing a second pressuresignal, processing means for processing the first and second pressuresignals to provide a processed signal, and means for providing athreshold signal, wherein the processing means further includescomparator means for comparing on the processed pressure signal with thethreshold signal and providing an output signal relating to thecomparison of the first and second processed signals.

Preferably, the monitoring arrangement comprises a pressure monitoringarrangement to measure the gas pressure at said first and secondregions. Desirably the output signal comprises a stall detection signalto indicate the presence or otherwise of the presence of stall.

The stall detection arrangement may comprise the further features of thestall detection means described above.

An embodiment of the invention will now be described by way of exampleonly, with reference to the accompanying drawings, in which:—

FIG. 1 is a diagrammatic view of a gas turbine engine; and

FIG. 2 is a schematic diagram showing the stall detection and recoverysystem.

Referring to FIG. 1, there is shown diagrammatically a two shaft gasturbine engine 1 comprising a fan 2, a fan intake duct 3, and an intakepressure probe 4 arranged in the intake duct 3. Downstream of the fan 2,the flow of air is divided to pass through a bypass duct 5 and a highpressure compressor 6.

The flow of air from the high pressure compressor enters a combustor 7where fuel is burned to drive the high and low pressure turbines 8.

Surge detection means comprising a combustor pressure probe 9 isarranged within the combustor 7 to measure the pressure therein. Thecombustor pressure probe 9 is connected to a stall detection andrecovery system 10.

Fuel supply injectors 11 are provided in the combustor 7 to supply fuelthereto. Also, the combustor 7 comprises igniters 12 for initialignition of the fuel in the combustor 7. The fuel injectors 11 and theigniters 12 are connected to the stall detection and recovery system.

The stall detection and recovery system 10 is adapted to detect enginesurge events by monitoring the combustor pressure via the combustorpressure probe 9. This technique is generally well known in the art andis not described herein. When a surge has been detected, the intake ofgas via the intake duct 3 is monitored by the intake pressure probe 4 todetect any

fluctuations in pressure, which would indicate that an engine stall hasoccurred.

A schematic diagram of the stall detection and recovery system 10 isshown in FIG. 2. A first pressure signal 20 is transmitted from theintake pressure probe 4 to processing means which comprises threeparallel filter blocks 21, 22, 23. The filter block 21 is a first orderfilter and produces a first filtered signal 24 indicative of the timeaverage value of the first pressure signal. The filter block 22 allowshigh input signals through, but applies a first order lag to lowsignals. The filter block 22 provides a second filtered signal 25indicative of the maximum peak level of the first pressure signal.

The filter block 23 allows low signals through, but applies a firstorder lag to high signals. The third filter block 23 provides a thirdfiltered signal 26 indicative of the minimum peak level of the firstpressure signal.

The filtered signals are fed to comparator means comprising first andsecond comparators 27, 28 to provide a first difference signal 29 whichis the difference between the first filtered signal 24 and the secondfiltered signal 25, and to provide a second difference signal 30 whichis the difference between the first filtered signal and the secondfiltered signal 26. The first and second difference signals 29, 30 arereceived by first gate means comprising a low wins gate 31, which allowsthe lowest of the first and second difference signals 29, 30 to betransmitted to multiplier means 32 where the signal is multiplied by apredetermined factor, which in the preferred embodiment is 2.0. Amultiplied signal 33 is provided from the multiplier means, and receivedby a divider means 34 in which the signal is divided by a processedsecond pressure signal 38, as explained below.

The multiplied signal 33 represents an approximation of the differencebetween the second pressure signal 25 and the third pressure signal 26.This has the advantage that the multiplied signal 33 is generallyunaffected or

minimally affected by sudden increased in fan intake pressure, of thesort that would be expected during rapid aircraft manoeuvres.

The combustor pressure probe 9 measures the pressure of the gas in thecombustor 8. A second pressure signal 35 is transmitted by the combustorpressure probe 9 to second processing means comprising second gate means36. The second gate means 36 is in the form of a high wins gate.

The control means 10 further includes a fixed signal transmission means37 to transmit a predetermined pressure signal 37A to the second gatemeans 36.

The predetermined pressure signal 37A is set to a fixed value, e.g. 10psia, which is lower than the second pressure signal 35 during normaloperation of the engine 1, and also lower than the second pressuresignal 35 during a stall. However, the fixed pressure signal is higherthan the combustor pressure during engine start or sub-idle operation orwhen it is unlit. This reduces the possibility of spurious stalldetection.

The second gate means 36 transmits the processed second pressure signal38 to the divider means 34 as described above. The signal transmitted bythe second gate means 36 is the higher of the second pressure signal 35and the fixed pressure signal 37A. In most situations, the signaltransmitted by the second gate means 36 will be the second pressuresignal 35. However, during engine start, or sub-idle operation of theengine or when the engine is unlit, the combustor pressure may be below10 psia, in which case the processed second pressure signal 38 will bethe fixed pressure signal 37A. The multiplied signal 33 provided by themultiplier means 32 is divided at the dividing means 34 by the processedsecond pressure signal 38 to provide a divided signal 39.

The divided signal 39 generally represents the multiplied signal 33expressed as a percentage of the processed second pressure signal 38.For example, during a stall, the multiplied signal 33 could be, forexample approximately 0.3 psia, and the combustor pressure, or theprocessed second pressure signal could be substantially 12 psia. In sucha situation the divided signal 39 would be calculated by:${\frac{0.3}{12} \times 100} = {2.5\%}$

The divided signal 39 is received by a comparator arrangement in theform of a further comparator 40 and compared to a threshold signal 41transmitted by threshold signal transmission means 42. The thresholdsignal 41 is also expressed as a percentage, and the threshold signal 41is typically in the region of 1.2%. Where the divided signal 39 isgreater than the threshold signal 41 i.e. during a stall, the outputsignal 44 provided by the further comparator 40 is a “true” signal or asignal of 1. During normal operation of the engine 1, the divided signal39 is less than the threshold signal 41 and the output signal 44provided by the further comparator 40 is a “false” signal or a signal of0.

The output signal 44 is transmitted to a fault integrator 46. The faultintegrator 46 is programmed to step up by a value of 0.2 each time itreceives an output signal 44 which has a value of 1.

In addition, the fault integrator 46 is programmed to step down by avalue of 0.2 each time it receives an output signal 44 which has a valueof 0.

When the fault integrator 46 has received sufficient output signals 44to have stepped up by a value of 1, the fault integrator transmits astall detection signal 48, which may be a signal having a value of 1.

In the embodiment shown, the surge detection means 50 receives acombustor pressure probe 9, indicating the pressure in the combustor 7.If the combustor pressure signal 52 can be the same as the secondpressure signal 35.

The details of surge detection are well known and are not describedherein. If the combustor pressure signal 52 indicates a surge, the surgedetection means 50 provides a surge detection signal 54 to delay means56.

The delay measuring 56 imparts a delay of substantially 0.5 s to thesurge detection signal 54 and transmits therefrom a delayed surgedetection signal 154.

The delayed surge detection signal 154 and the stall detection signal 48are transmitted to stall recovery means 58. If the stall recovery means58 is still receiving a stall detection signal 48 when the delayed surgedetection signal 154 is received thereby, the stall recovery means 58commences stall recovery action by providing a stall recovery signal orsignals 60.

The stall recovery signal or signals 60 can be transmitted to, forexample, the fuel system to momentarily interrupt the fuel supply fed tothe injectors 11. The stall recovery signals 11 can also be transmittedto the igniters 9 to ignite the fuel after the momentary interruption.

There is thus described a system for detection of a stall in a gasturbine engine and for recovery therefrom. The system has the advantageof only taking action to recover from the stall in the event thatnatural recovery will not occur. The preferred embodiment provides adirect indication of the presence of stall by utilising the fluidmechanical processes intrinsic to the stall itself, which provide anindication of the presence of stall. This is an advantage over prior artprocesses which rely on commonly observed side effects from stalls,which may either not be present or may be caused by other

phenomena. Moreover, the system utilises pressure measurement at the fanintake duct which has the advantage of being essentially pseudo-steadyduring normal engine operation.

Various modifications can be made without departing from the scope ofthe invention. For example, the system could be applied to enginepressure, other than the pressure at the fan intake duct if account istaken of the variation of these pressures during normal engineoperation.

Whilst endeavouring in the foregoing specification to draw attention tothose features of the invention believed to be of particular importanceit should be understood that the Applicant claims protection in respectof any patentable feature or combination of features hereinbeforereferred to and/or shown in the drawings whether or not particularemphasis has been placed thereon.

1. A stall detection and recovery system for a gas turbine engine, thesystem comprising surge detection means for detecting an engine surgeand proving a surge detection signal for indicating said engine surge,stall detection means to provide a stall detection signal relating to acondition of gas flowing through the engine, and stall recovery means toreceive said surge detection signal and said stall detection signal,said stall recovery means being arranged to control combustor operatingmeans whereby when the stall detection signal indicates an engine stall,the stall recovery means controls the combustor operating means toeffect recovery from the stall.
 2. A system according to claim 1 whereinthe combustor operating means comprises means to supply fuel to thecombustor, and an ignitor for igniting the fuel.
 3. A system accordingto claim 1 wherein the stall recovery means operates to interruptmomentarily said supply of fuel and to ignite the fuel supplied to theengine after said momentary interruption.
 4. A system according to claim1 wherein the condition of the gas to which the stall detection signalrelates is the pressure of the gas.
 5. A system according to claim 1including delay means to impart a delay of a predetermined time to thesurge detection signal, whereby the stall recovery means operates toeffect recovery from the stall a predetermined time after the surgedetection signal indicates an engine surge providing the stall detectionsignal still indicates an engine stall.
 6. A system according to claim 5wherein the predetermined time is in the region of 0 to 10 seconds.
 7. Asystem according to claim 6 wherein the predetermined time is in theregion of 0.1 to 10 seconds.
 8. A system according to claim 7 whereinthe predetermined time is in the region of 0.1 to 1 second.
 9. A systemaccording to claim 8 wherein the predetermined time is substantially 0.5seconds.
 10. A system according to claim 5 wherein the surge detectionmeans provides a surge signal, indicative of a surge having beendetected, and the surge signal is received by the delay means whichimparts said delay to the surge signal.
 11. A system according to claim10 wherein the surge detection means comprises combustor monitoringmeans to monitor a condition of the gas in the combustor.
 12. A systemaccording to claim 11 wherein the combustor monitoring means monitorscombustor pressure.
 13. A system according to claim 1 wherein the stalldetection means comprises a pressure monitoring arrangement formonitoring the pressure of the gas at a region of the engine andproviding a pressure signal.
 14. A system according to claim 13 whereinthe stall detection signal and the surge detection signal aretransmitted through the stall recovery means for controlling the supplyof fuel and the ignitor as aforesaid.
 15. A stall detection system for agas turbine engine, comprising a monitoring arrangement for monitoringthe condition of gas flowing through a first region of the engine andproviding a first signal relating to said condition of the gas at thefirst region, and for monitoring the condition of gas flowing throughthe engine at a second region of the engine and providing a secondsignal, processing means for processing the first, and second signals toprovide a processed signal, means for providing a threshold signal, theprocessing means further including a comparator means for comparing onthe processed signal with the threshold signal and providing an outputsignal relating to the comparison of the first and second processedsignals.
 16. A system according to claim 15 wherein the monitoringarrangement comprises a pressure monitoring arrangement for monitoringthe pressure of the gas of said regions, whereby the first and secondsignals are first and second pressure signals.
 17. A system according toclaim 11 and further including processing means to process at least oneof the pressure signals.
 18. A system according to claim 11 wherein thepressure monitoring arrangement comprises a first pressure measuringmeans for measuring the pressure at a first region of the engine andproviding a measured first pressure signal, being said pressure signal,and a second pressure measuring means for measuring the pressure at asecond region of the engine and providing a measured second pressuresignal.
 19. A system according to claim 18 wherein the second region ofthe engine is the combustor, and the measured second pressure signalrelates to the pressure in the combustor.
 20. A system according toclaims 18 wherein the processing means comprises filter means forfiltering the first pressure signal, the filter means providing a firstfiltered signal indicative of the time average value of the firstpressure signal, a second filtered signal indicative of high values ofthe first pressure signal, and a third filtered signal indicative of lowvalues of the first pressure signal.
 21. A system according to claim 20wherein the second filtered signal is indicative of maximum peaks of thefirst pressure signal, and the third filtered signal is indicative ofminimum peaks of the first pressure signal.
 22. A system according toclaim 17 wherein the processing means further includes comparator meansfor providing a first difference signal, being the difference betweenthe first filtered signal and the second filtered signal, and forproviding a second difference signal, being the difference between thefirst filtered signal the third filtered signal.
 23. A system accordingto claim 22 wherein the processing means further includes gate means,for transmitting the lowest of the first and second difference signals.24. A system according to claim 23 wherein the processing means furthercomprises multiplying means for multiplying said transmitted differencesignal by a predetermined factor and provide a multiplied pressuresignal.
 25. A system according to claim 24 wherein the predeterminedfactor is
 2. 26. A system according to claim 24 comprising fixed signaltransmission means for transmitting a predetermined second pressuresignal.
 27. A system according to claim 26 wherein the predeterminedsecond pressure signal is a signal relating to a pressure which is lowerthan the expected pressure of the second pressure signal during normalrunning of the engine, and higher than the pressure in the second regionof the engine during engine start or sub-idle operation or when theengine is unlit.
 28. A system according to claim 26 further includingsecond processing means to process the second measured pressure signaland the predetermined second pressure signal, the second processingmeans comprising further gate means for transmitting the highest of themeasured second pressure signal and the predetermined second pressuresignal.
 29. A system according to claim 28 wherein the stall detectionmeans comprises divider means to divide the multiplied pressure signalby the transmitted second pressure signal to provide a divided signal.30. A system according to claim 29 wherein the stall detection meansfurther comprises a comparator device to compare the divided signal witha threshold signal, whereby in the event of the divided signal beinggreater than the threshold signal, the comparator device provides astall output signal.
 31. A system according to claim 30 wherein thestall detection means comprises integration means to receive the stalloutput signal, the integration means producing said stall detectionsignal which indicates that a stall has been detected.
 32. A systemaccording to claim 31 wherein the integration means is a faultintegrator.
 33. A stall detection and recovery method for a gas turbineengine, the method comprising detecting an engine surge and proving asurge detection signal for indicating said engine surge, detecting anengine stall and providing a stall detection signal relating to acondition of gas flowing through the engine, and providing a stallrecovery means to receive said surge detection signal and said stalldetection signal, said stall recovery means being arranged to controlcombustor operating means whereby when the stall detection signalindicates an engine stall, the stall recovery means controls thecombustor operating means to effect recovery from the stall.
 34. Amethod according to claim 33 wherein the combustor operating meanscomprises means to supply fuel to the combustor, and an ignitor forigniting the fuel.
 35. A method according to claim 32 wherein therecovery from stall comprises interrupting momentarily said supply offuel and to ignite the fuel supplied to the engine after said momentaryinterruption.
 36. A method according to claim 33 wherein the conditionof the gas to which the second signal relates is pressure of gas.
 37. Amethod according to claim 33 wherein a delay of a predetermined time isimparted to the surge detection signal, whereby the stall recovery meansoperates to effect recovery from the stall said predetermined time afterthe surge detection signal indicates an engine surge providing the stalldetection signal still indicates an engine stall.
 38. A method accordingto claim 37 wherein the predetermined time is in the region of 0 to 10seconds.
 39. A method according to claim 38 wherein the predeterminedtime is in the region of 0.1 to 10 seconds.
 40. A method according toclaim 39 wherein the predetermined time is in the region of 0.1 to 1second.
 41. A method according to claim 40 wherein the predeterminedtime is substantially 0.5 seconds.
 42. A method according to claim 37wherein a surge signal is provided, indicative of a surge having beendetected, said delay is imparted to the surge signal.
 43. A methodaccording to claim 33 including monitoring the pressure of the gas at aregion of the engine and providing a pressure signal.
 44. A methodaccording to claim 43 comprising providing combustor monitoring means tomonitor a condition of the gas in the combustor.
 45. A method accordingto claim 44 wherein the combustor monitoring means monitors combustorpressure.
 46. A method according to claim 45 wherein the stall detectionsignal and the surge detection signal are transmitted through thecontrol means for controlling the supply of fuel and the ignitor asaforesaid.
 47. A stall detection method for a gas turbine engine,comprising monitoring the condition of gas flowing through a firstregion of the engine and providing a first signal relating to saidcondition of the gas at the first region, monitoring the condition ofgas flowing through the engine at a second region of the engine andproviding a second signal, processing the first, and second signals toprovide a processed signal, providing a threshold signal, comparing theprocessed signal with the threshold signal and providing an outputsignal relating to the comparison of the first and second processedsignals.
 48. A method according to claim 47 wherein the conditionmonitored is the pressure of the gas at said region.
 49. A methodaccording to claim 48 further including processing the pressure signal.50. A method according to claim 48 wherein the step of monitoring thepressure comprises measuring the pressure at a first region of theengine and providing a measured first pressure signal, being saidpressure signal, and measuring the pressure at a second region of theengine and providing a measured second pressure signal.
 51. A methodaccording to claim 50 wherein the second region of the engine is thecombustor, and the measured second pressure signal relates to thepressure in the combustor.
 52. A method according to claim 50 includingfiltering the first pressure signal, to provide a first filtered signalindicative of the time average value of the first pressure signal, asecond filtered signal indicative of high values of the first pressuresignal, and a third filtered signal indicative of low values of thefirst pressure signal.
 53. A method according to claim 52 wherein thesecond filtered signal is indicative of maximum peaks of the firstpressure signal, and the third filtered signal is indicative of minimumpeaks of the first pressure signal.
 54. A method according to claim 49comprising providing a first difference signal, being the differencebetween the first filtered signal and the second filtered signal, andproviding a second difference signal, being the difference between thefirst filtered signal the third filtered signal.
 55. A method accordingto claim 54 including transmitting the lowest of the first and seconddifference signals.
 56. A method according to claim 55 includingmultiplying said transmitted difference signal by a predetermined factorto provide a multiplied pressure signal.
 57. A method according to claim56 wherein the predetermined factor is
 2. 58. A method according toclaim 56 including transmitting a predetermined second pressure signal.59. A method according to claim 58 wherein the predetermined secondpressure signal is a signal relating to a pressure which is lower thanthe expected pressure of the second pressure signal during normalrunning of the engine, and higher than the pressure in the second regionof the engine during engine start or sub-idle operation or when theengine is unlit.
 60. A method according to claim 58 further includingprocessing the second measured pressure signal and the predeterminedsecond pressure signal, and transmitting the highest of the measuredsecond pressure signal and the predetermined second pressure signal. 61.A method according to claim 22 including dividing the multipliedpressure signal by the transmitted second pressure signal to provide adivided signal.
 62. A method according to claim 61 including comparingthe divided signal with a threshold signal, whereby in the event of thedivided signal being greater than the threshold signal, a stall outputsignal is provided.
 63. A method according to claim 62 includingtransmitting the stall output signal to integration means to receive thestall output signal, the integration means producing said stalldetection signal which indicates that a stall has been detected.
 64. Amethod according to claim 63 wherein the integration means is a faultintegrator.
 65. A gas turbine engine incorporating a system according toclaim 1.